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Fig. 1 6. Diagram to illustrate the possible ways in which the membrane 

 potential changes with time after the appHcation of a steady depolarizing 

 current. (A) (hypothetical) No local membrane response is triggered; 

 potential rises to level of spike threshold (Vth) along an exponential 

 curve dictated solely by the membrane time constant. The first and 

 succeeding impulse intervals are longer than the interval between 

 stimulus onset and the first spike, since membrane refractoriness (dotted 

 line) raises impulse threshold following each transient. (B) As first 

 shown by Hodgkin, impulse threshold is reached by the growth of a local 

 or graded membrane response, the threshold for which is indicated by the 

 line Vgr. Following an instantaneous onset of constant current, the 

 membrane potential at first changes in an exponential fashion until it 

 reaches Vgr. The local response then brings the level to impulse threshold. 

 If membrane refractoriness is brief, no interference with membrane 

 excitation occurs, and succeeding spike intervals are identical with the 

 interval between stimulus onset and the first spike. (C) Differences 

 between membrane potential changes occasioned by purely passive 

 electrical characteristics (dotted lines), and a combination of passive and 

 active factors (solid lines). Responses to four different levels of stimu- 

 lating cvurent are shown. (From Fuortes and Mantegazzini," Fig. i.) 



43 



